Two major classes of engines are used to convert heat energy to mechanical energy and/or electrical energy—these being internal combustion (IC) and external combustion (EX) engines. Internal combustion engines dominate the transportation industry while the major applications of external combustion engines are found in the power generation industry where steam powered turbines are still a major application of the external combustion principle.
Stirling engines (SE) are external combustion engines with higher energy density than piston-based steam engines that may be as energetically efficient as internal combustion engines. Like steam power, SE's suffer relative to IC engines in having less dynamic power output; thus they are commonly found in applications where the power demand is relatively constant. The SE is a thermodynamic engine that delivers power by alternatively heating and cooling a fixed volume of gas with work being done by the pressure increase during the heating phase. A number of arrangements for achieving the alternate heating and cooling of the working fluid (i.e. a gas) have been developed, giving rise to three main forms of the engine (alpha, beta and gamma). In these traditional configurations and commercialized arrangements of a SE, the mechanical work is usually produced by the pressure of the heated gas acting on piston-crankshaft arrangements. The heat exchange surface is the surface of the cylinder(s) but mostly the cylinder head(s). Rotating SE's with crankshaft/piston designs require special seals, or provision to regenerate and recharge the working gas as it is lost through the joints provided for lubrication and power transfer.